Many engineering pipe specifications will specify the use of 300# flanges for pipe diameters of 3" and below for pipe classes specified for 150# service.  Does anyone know the basis of this?  I have heard it is to provide the smaller pipe more structural strength and resistance to vibration issues. 

There is no basis for it other than to provide extra mechanical strength when needed.  I think its over the top to provide the next higher class rating and would prefer to do something like specifying XXS pipe as a minimum and increase the bore of the flanges to match, when such piping is to be used to fabricate a gas piping blowdown vent, or similar application where stresses are likely to be higher than the norm.  Maybe that's not likely to carry the higher moments that a extra class flange would handle, but the piping is likely to be the weakest link-stress wise. If you're trying to stop leaks with an extra class flange, I would think it would just cause a nearby 6" flange gasket to blow first. No gain there. >filter< Asking for extra class factor, without necessarily specifying a greater wall thickness is not likely to achive the desired effect anyway, as the pipe with a wall thickness matching the pressure requirement might not be able to transfer the full moment of a flange that has been increased one class higher.  Where's the advantage of specifying one component to a higher standard, when the connected pipe is not speced in the same manner.

A lot of offshore pipelines in the GOM in the early 80s arbitrarily used 900# class where only 600# was required and 600# where only 300# was needed.  Enron was one of them.  I got them started using the proper flanges with no ill effects that I've ever heard of.  I think even Transco changed after that.  I follow the logic I've already mentioned above, again with no ill effects that I'm aware of, then again why put a 2" pipe somewhere a bolt is going to get knocked off?  Seems like the pipe and flange would be damaged before you kicked off a bolt without hurting those.  Leaves me to believe that the gasket integrity was the only worry.  Well, I guess its only a 2", so whatever floats your boat....

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